The control of quality of basic steel products such as bars, rods, plates, sheets and the like has become of increasing importance in recent years. One factor has been a greater demand for materials having high strength-to-weight ratios, for use in vehicles and other products. Another factor has been in the growing recognition of the waste which results when a purchaser of a basic steel product invests a substantial amount of time, materials and effort in manufacture of an end product and then finds it necessary to reject the end product after a final inspection because of a defect which existed in the basic steel product. Even more serious problems result when an end product contains a defect which is not found until later when the product is in use.
Certain very troublesome types of defects are encountered in connection with roller operations which are used in making a variety of steel products. After first casting steel as ingots, rolling operations are used to roll the ingots into slabs and to then roll the slabs into plates and sheets. Rolling operations are also used to roll ingots into blooms, to then roll the blooms into billets and to then roll billets into bars, rods and similar rolled products which may be used by manufacturers to make a great many different types of final products.
It is found that the slabs, blooms and billets initially rolled from ingots frequently contain longitudinal cracks in or near the surface thereof and that such cracks are not removed in subsequent rolling operations. On the contrary, each crack is usually increased in length in subsequent rolling operations and may cause a long length of the final rolled product to be defective.
In the prior art, various procedures have been used to detect cracks and to remove the surface portions of billets or other raw products which contain such cracks. To detect cracks, magnetic particle inspection has been used very successfully, magnetic particle inspection being very sensitive to cracks in or near the surface of a part. After detection of cracks, the portions which contain the cracks may be removed by using torches, by scarfing and milling procedures and by grinding procedures. Grinding procedures have been used increasingly in recent years and are highly effective so that the quality of the final rolled product can be substantially enhanced. However, a very high degree of skill and attention is required on the part of the grinding equipment operator, working in noisy and dirty conditions with poor visibility. It is very difficult to obtain a high degree of assurance that the billet or other raw product will be completely free of defective portions.
It is noteworthy that continuous casting procedures may be used in a manner such as to avoid certain problems connected with rolling procedures. However, the construction of plants for continuous casting involves huge capital expenditures and there exists a very large investment in rolling equipment which would be lost if an attempt were made to substitute continuous casting procedures for rolling procedures. Moreover, casting procedures are not without problems and rolling procedures have certain inherent advantages over casting procedures in the production of many types of products.